Chromates are widely referred to in the literature, and their effectiveness in reducing the rate of corrosion of metallic surfaces exposed to corroding aqueous mediums is well known. However, if used in low concentrations these inhibitors can cause considerable pitting and tuberculation. Indeed, if added in insufficient quantities to stop metal surface attack altogether, corrosion can become so severely localized and the intensity of attack so intense that perforations may occur more extensively than if no inhibitor treatment had been added.
In Corrosion Inhibitors, 3rd edition; National Association of Corrosion Engineers; Houston, Tex. (1977); p. 134, it is stated that the critical concentration for passivation by chromate in distilled water is 81 to 162 ppm and that the critical level for chromate increases as chloride and sulfate levels increase. Present practice in industrial water systems is to employ chromates alone in concentrations no lower than 200 ppm.
Economic and environmental considerations make it desirable to minimize chromate levels in water. This goal has been attained by combining the chromate with other compounds such as phosphate and zinc. For example, U.S. Pat. No. 2,711,391 to Kahler discloses the use of water-soluble chromate in combination with water-soluble phosphate. According to this reference, the combined treatment permits the use of lower levels of chromate than had theretofore been used. However, the lowest dosage rate for chromate achievable, according to Kahler, is 5 ppm. In fact, the reference specifically sets the preferred lower limit for chromate at 11 ppm. U.S. Pat. No. 2,900,222 to Kahler et al teaches the use of chromate compound at levels as low as 1 ppm, but only in combination with both water-soluble orthophosphate and water-soluble zinc compound. A problem related to supplemented chromate treatments such as those disclosed in the Kahler references is that the discharge of industrial waters containing heavy metals and/or phosphorous compounds is under strict environmental control. The quality of water discharged to natural waters is presently being regulated and is expected to be under greater scrutiny in the future.